How do drugs change human information processing (HIP) and what neuropharmacological systems mediate these effects? There are three general models that may account for the effects of drugs on HIP. The first accounts for changes in HIP by postulating that specific neurotransmitter operations drive specific cognitive processes. The second proposes that the cognitive process driven by a given NT operation changes with the amount of NT and thus is influenced with dose and time. A third, holds that most NI operations act on a single resource, so that different NT operations cannot be linked to separate cognitive processes. Predictions from these models can be operationalized and tested using a model of information processing is has been volidated with stimulants. In the present proposal, the predictions from these models will be tested in two experiments. In the first, we will examine the effects of the noradrenergic drugs cloniding and yohimbine. In the second, we will examine the dopsminorgic drugs tromocriptine and pimozide. Information processing is assessed by reaction time supplemented by the legency of P300 and laterslized motor potentials (LMP) of the scalp even related potential (ERP). These dependent varieties are measured in tasks with well studied manipulations that have produced replicate drug effects in several experiments. P300 and the LMP is measured using a newly developed procedure in which ERP topological features are used to identify component latency. The effects of drug (agonist and antagolist), dose and time of testing are used along with task variables to compare predictions generated from these models. The outcome of these studies will have important implications for understanding cognitive changes in depression and other psychiatric disorders.